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 A271576 Integers whose square is of the form x^2 + y^4, with x,y > 0 (see A111925). 16
 5, 15, 20, 34, 39, 41, 45, 60, 65, 80, 85, 111, 125, 135, 136, 145, 150, 156, 164, 175, 180, 194, 219, 240, 245, 255, 260, 265, 306, 313, 320, 325, 340, 351, 353, 369, 371, 375, 405, 410, 444, 445, 455, 500, 505, 514, 540, 544, 580, 585, 600, 605, 609, 624, 629, 656, 671, 674, 689 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If z is in this sequence, z*k^2 is, for k > 0. Also note that since there are no fourth powers in A111925, there are no squares in this sequence. - Altug Alkan, Apr 10 2016 From Karl-Heinz Hofmann, Oct 22 2021: (Start) Terms must have at least one prime factor of the form p == 1 (mod 4), a Pythagorean prime (A002144). If the terms additionally have prime factors of the form p == 3 (mod 4), which are in A002145, then they must appear in the prime divisor sets of x and y too. (End) From Jon E. Schoenfield, Nov 15 2021: (Start) Apparently, the vast majority of the terms of this sequence can be expressed as x^2 + y^4 with x,y > 0 in only one way (A345645 lists those terms), but some can be so expressed in exactly two, three, four, five, six, or more ways. Among the first 3976926961 terms of this sequence, 3948648229 are in A345645 (exactly 1 way), 25415062 are in A345700 (exactly 2 ways), 2697713 are in A345968 (exactly 3 ways), 161543 are in A346110 (exactly 4 ways), 3989 are in A348655 (exactly 5 ways), 424 are in A349324 (exactly 6 ways), and just 1 -- a(3976926961) = 2474052064291275 = A346115(7) -- is a number whose square can be written as x^2 + y^4 with x,y > 0 in exactly 7 ways. The ratios of successive counts above, i.e., 3948648229/25415062, 25415062/2697713, 2697713/161543, 161543/3989, 3989/424, and 424/1, are approximately 155.4, 9.4, 16.7, 40.5, 9.4, and 424.0. What is it that (over the interval [1, 2474052064291275], at least) makes numbers whose squares can be written as x^2 + y^4 with x,y > 0 in more than 6 ways so much rarer than those that can be written thus in exactly 6 ways? (End) LINKS Chai Wah Wu, Table of n, a(n) for n = 1..10000 EXAMPLE 5^2 = 25 = 9 + 16 = 3^2 + 2^4, so 5 is a term. MATHEMATICA Select[Range@ 200, Resolve[Exists[{x, y}, Reduce[#^2 == x^2 + y^4, {x, y}, Integers], And[x > 0, y > 0]]] &] (* Michael De Vlieger, Apr 10 2016 *) PROG (PARI) isok(n) = n = n^2; for (k=1, n-1, if (issquare(k) && (p=ispower(n-k, 4)), return (1))) (PARI) is(n)=my(n2=n^2); for(b=sqrtnint(2*n-2, 4)+1, sqrtint(n-1), if(issquare(n2-b^4), return(1))); 0 \\ Charles R Greathouse IV, Nov 16 2021 CROSSREFS Cf. A111925, A271577, A345645 (in exactly 1 way). Cf. A345700 (in exactly 2 ways), A345968 (in exactly 3 ways). Cf. A346110 (in exactly 4 ways), A349324 (in exactly 6 ways), A346115 (the least solutions). Cf. A002144 (p == 1 (mod 4)), A002145 (p == 3 (mod 4)). Sequence in context: A290551 A045176 A273908 * A345645 A274535 A027184 Adjacent sequences: A271573 A271574 A271575 * A271577 A271578 A271579 KEYWORD nonn AUTHOR Michel Marcus, Apr 10 2016 STATUS approved

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Last modified May 21 19:35 EDT 2024. Contains 372738 sequences. (Running on oeis4.)